This invention relates to electro-optic displays.
Many computer displays have a limited physical resolutionxe2x80x94typically 70-100 dots per inch (2.76 to 3.94 dots per mm). Since the display is composed of an array of rectangular pixels, each of which is either ON or OFF, the edges of text, lines, etc. that are displayed may often have a jagged xe2x80x9cstaircasexe2x80x9d effect which is visually disturbing.
There have been several attempts to solve this problem in a variety of ways e.g. by grey scale rendering or anti-aliasing. Some display technologies such as Twisted Nematic Liquid Crystal Displays (TN LCDs), cathode ray tubes, etc., allow the intensity of the whole area of a pixel to be varied. In these types of display, the intensity of each pixel may be selected in proportion to the area of the pixel that should be ON. Whilst this can reduce the visually disturbing staircase effect, it can make edges in the displayed image appear blurred, especially when viewed from close to the display.
Other display technologies do not allow a range of intensities over the whole area of the pixel display, in which case a range of intensities may be simulated by rapidly turning the pixel ON and OFF, sufficiently fast so that the eye sees the average intensity. This may be used with Super Twisted Nematic (STN) LCDs. Alternatively, each pixel may be divided into sub pixels, and a varying number turned on according to the desired intensity. The display must then be viewed from such a distance that the eye cannot resolve the sub pixels, or some optical blurring introduced to average out the intensity over the whole pixel. An example of this type of technique is described in JP-A-3142260, where each pixel is effectively divided into four sub pixel slices. The image to be displayed is analysed and two-bit pixel data is added to each pixel to turn on selected sub pixel slices during a pixel sub-scanning period. This allows a range of intensities to be displayed by varying the area of the pixel that is ON, in four discrete steps. However, by its nature this system is only capable of modulating the pixel output slice-wise and in many instances this will not give good smoothing, particularly where the edge to be smoothed is nearly perpendicular to the slice direction of the pixels.
A similar technique is disclosed in U.S. Pat. No. 4,824,218, which relates principally to Ferroelectric LCDs. Here a variable width portion of a pixel is turned on by driving a potential gradient across the width of the pixel by means of metal electrodes running along the edges of resistive transparent column electrodes. To allow the complete area of the pixel to be driven, whilst preventing crosstalk (i.e. unintentionally affecting other pixels in the same row), and to avoid a wasted area of the pixel nearest the xe2x80x9creferencexe2x80x9d metal electrode, the pixel is driven in two phases, swapping the role of the two metal electrodes between xe2x80x9creferencexe2x80x9d and xe2x80x9cdataxe2x80x9d. This technique relies on the fact that a Ferroelectric Liquid Crystal (FLC) material stores its state and can be written to again, adding to the area that has already been turned ON, which is not true for all bistable materials. This scheme also requires a blanking pulse to clear the whole pixel before the two writing phases. U.S. Pat. No. 4,824,218 also refers to an extension of this technique in which the display has row and column transparent resistive electrodes each with metal electrodes on either side. A four field drive scheme is described in which the display is scanned four times, following a blanking pulse, to make up a frame. As before the metal electrodes swap roles between data and reference electrodes. Because alternate electrodes are set to a fixed reference this arrangement does not allow great flexibility in the creation of sub pixel shapes.
Accordingly, in one aspect, this invention provides a display comprising:
a first substrate having thereon a plurality of row electrode means,
a second substrate having thereon a plurality of column electrode means,
a layer of electro-optic material disposed between said first and said second substrate,
row drive means associated with said first substrate for applying a respective selected voltage profile across each row electrode means in a direction transverse to the thickness of said layer,
column drive means associated with said second substrate for applying a respective selected voltage profile across each column electrode means in a direction transverse to the thickness of said layer,
whereby the electrical field in each pixel in the direction transverse to the thickness of the electro-optic layer may be selected to provide a non-uniform optical output.
In a particularly preferred arrangement, each of said column electrode means and said row electrode means comprises a group of generally parallel conductive tracks.
We have found that the multiple track architecture for each of the row and column electrodes provides important and unexpected advantages when used in conjunction with drive means which apply a selected voltage profile across each of the groups of tracks making up an electrode. In this way the magnitude of the electrical field across the pixel may be varied in a direction transverse to the thickness of the electro-optic layer to provide a non-uniform optical output across the pixel. By contrast to the arrangements of U.S. Pat. No. 4,824,218, which use resistive electrodes driven by metal electrodes to either side, the multiple conductive tracks of the present invention may be driven by electrical contacts well away from the image area, thus considerably improving the aperture ratio of the display. Also, the previous arrangement of U.S. Pat. No. 4,824,218 requires accurate alignment of fine metal electrodes with each of the transparent resistive electrodes, whereas in the present invention the conductive tracks in each group may be coupled together by a resistive element in contact with the end regions of the conductive tracks to one side of the display, and an input electrode provided at each end of the resistive element. Here the accuracy of alignment of the tracks and the resistive elements is not so important as the resolution of the conductive tracks may be effectively decoupled from that of the resistive elements.
Thus, in one embodiment, the resistive elements may be formed on the first and second substrates, in electrical contact with the respective conductive tracks. Alternatively, the row and column resistive elements may be formed on separate substrates which are then placed in contact with the first and second substrates.
For both the rows and the columns, the series of resistive elements driving the groups of conductive tracks may be replaced by a single resistive element in electrical contact with a substantial proportion of, or all the conductive tracks making up the complete set of row/column electrodes, with the drive means including an input electrode means between each group of conductive tracks.
Preferably, said drive means includes means for applying an adjustable voltage across each of said resistive elements, so that the voltage profile across the group of conductive tracks is a ramp of positive, negative or zero slope. Instead of resistance coupling, the drives to the groups of tracks may be inductively or capacitively coupled.
By the above arrangements, the electrical field may be configured to generate a wide range of different non-uniform outputs of selected shape for a pixel, to turn on an arbitrary portion of the pixel, so that the required edge of the line portion of the text character etc. is maintained within the area of the pixel.
In one preferred drive scheme, each of a set of predetermined voltage profiles is applied across a row electrode means in successive phases, and the columns driven in parallel with the required voltage profiles. In this way a broad range of pixel shapes may be provided in either a single write (i.e. just one of the phases) or a multiple write where the pixel output is incrementally rendered. It will be appreciated that this drive scheme could be modified so that the multiple successive phases are applied to the columns whilst the rows are driven in parallel.
The electro-optic material may have a steep or xe2x80x9cfastxe2x80x9d electro-optic curve, i.e. where the electro-optic effect switches state abruptly at a particular threshold voltage, so that the optical output at a particular point within the pixel will vary between two generally discrete levels, dependent on whether the field strength at that point is above or below the threshold voltage. In general the boundary between ON and OFF regions in the pixel will be determined by an equi-potential line on the notional voltage surface within the pixel of magnitude corresponding to said threshold voltage. Alternatively, the electro-optic material may have a shallow or xe2x80x9cslowxe2x80x9d electro-optic curve, with the output varying continuously between ON and OFF through grey levels, so that the optical output may be shaded across the pixel, generally in accordance with the magnitude of the electrical field.
We have found that the technique of providing modulation of the voltage field in a direction transverse to the thickness of the electro-optic layer may be extended to other forms of display, again to provide a non-uniform optical output, with remarkable results.
Thus, in another aspect this invention provides a display comprising:
first and second substrate means provided to either side of a layer of electro-optic material,
the first substrate having thereon at least one electrically resistive surface,
electrode means for applying selected voltages to respective points or regions across said resistive surface, and
drive means for applying selected voltages to said electrodes to provide across a pixel an electrical field whose magnitude varies in at least one direction transverse to the thickness of the layer of electro-optic material, thereby to provide a non-uniform optical output across said pixel.
The first substrate may have an array of discrete electrically resistive surface elements each defining a pixel, with each surface element having three or more electrodes attached thereto. Thus, three or more selected voltages may be applied to spaced points or regions of said discrete resistive layer element, for example a respective voltage at each corner of a rectangular pixel. In this arrangement, each pixel effectively interpolates the four corner voltage values around the pixel and, for an electro-optic material with a steep curve, at each point within the pixel where the voltage passes the threshold value, the optical output changes.
We have also found that the ability to vary the optical output across a pixel instead of uniformly over the whole pixel means that we can provide displays which provide an effect similar to the half-tone type of process. In one arrangement the image is made up of pixels each containing a dot of controllably variable size from zero to fully filling the pixel, or even extending beyond its nominal boundary. Thus a display may comprise an electrode array of resistive electrodes with a plurality of connection points whereby an image may be created by a display of variable size dots.
The electro-optic material may comprise a liquid crystal material such as twisted nematic, supertwist nematic, polymer dispersed liquid crystal (PDLC), or ferro-electric materials. Alternatively it may comprise other electro-optic materials or devices, such as field emissive devices.
In operation of a typical example, the image to be displayed is analysed to determine those pixels where some form of intra-pixel variation is required, for example where the pixel is lying on an edge of the image or where grey-scaling or half toning is required. For each such pixel, the display driver determines the desired boundary lines between ON and OFF, and the electrical field intensity required, and then selects suitable voltages to be applied to the pixel to generate the required variation or level of electrical field across the pixel.
Whilst the invention has been defined above, it extends to any inventive combination of the features set out above or in the following description.